1. Field of the Invention
This invention relates generally to vehicle transparencies, such as vehicle windshields, and, in one particular embodiment, to a heatable vehicle windshield.
2. Technical Considerations
It has been known to pass direct current (DC) through a conductor (e.g., a wire or an electrically conductive coating) on a laminated vehicle windshield in order to raise the temperature of the windshield. This is particularly useful in colder climates for defogging and to melt ice and snow on the windshield.
In wire-heated windshields, fine electrically-conductive wires are placed between the windshield plies. The wires are connected to the output of a conventional vehicle alternator to supply direct current (DC) electricity to the wires. The wires have sufficiently low resistance to provide the windshield with a power density of 5 to 7 watts per decimeter squared (W/dm2).
However, a problem with wire-heated windshields is that the wires can be seen by the vehicle occupants. The presence of these wires can be aesthetically undesirable and can interfere with visibility through the windshield. If the diameter of the wires is decreased to try to reduce the visibility of the wires, the number of wires must be increased to maintain the desired power density. The increased number of wires adversely decreases the total solar energy transmitted (TSET) of the windshield.
In an effort to address this problem, some heated windshields utilize transparent, conductive coatings rather than wires to heat the windshield. While these known transparent coatings overcome the aesthetic and visibility problems associated with the use of wires, windshields heated by these coatings also have some drawbacks. For example, conventional heated windshield coatings typically have a sheet resistance of 2 ohms per square (Ω/□) or greater. A conventional vehicle alternator (14 volt (v) DC; 80 ampere; 1,120 watts) does not provide sufficient voltage to heat the windshield to a temperature sufficient for de-icing. Therefore, for vehicles utilizing conductive coatings rather than conductive wires, the vehicles must be altered to increase the available DC voltage. One way to do this is to replace the conventional alternator with an alternator that supplies more direct current output, such as a 42 v DC (2,500 W to 5,000 W) alternator. Another way to address this problem is to maintain the conventional 14 v DC alternator but add a DC to DC converter to step up the direct current voltage from the alternator to a sufficient level, e.g., 42 v DC, to heat the windshield to a commercially acceptable level.
However, replacing the conventional alternator or adding a DC to DC converter increases the cost and complexity of the vehicle electrical system.
Another solution has been to divide the coating into several electrically isolated sections and supply DC power to each individual section. However, this also increases the cost and complexity of the windshield.
Therefore, it would be desirable to provide a transparency, such as a laminated vehicle transparency, that reduces or eliminates at least some of the problems associated with conventional heatable transparencies.